In a manner known per se, a photovoltaic generator (PVG) includes one or more photovoltaic (PV) cells connected in series and/or in parallel. In the case of inorganic materials, a photovoltaic cell essentially comprises a (pn or pin junction) diode composed on the basis of a semiconductor material. This material has the property of absorbing light energy, a significant part of which can be transferred to charge carriers (electrons and holes). The constitution of a (pn or pin junction) diode by the doping of two zones of type N and type P respectively—possibly separated by a non-doped region (referred to as “intrinsic” and designated by “i” in the pin junction)—allows the charge carriers to be separated for them then to be collected via electrodes which the photovoltaic cell comprises. The maximum potential difference (open-circuit voltage, VOC) and the maximum current (short-circuit current, ICC) that the photovoltaic cell can supply are a function of both the materials making up the cell as a whole and the conditions surrounding this cell (including illumination via the spectral intensity, temperature, etc.). In the case of organic materials, the models are markedly different—making further reference to the notion of donor and acceptor materials in which electron-hole pairs known as excitons are created. The objective remains the same: to separate the charge carriers to collect and generate a current.
FIG. 1 shows schematically an example of a photovoltaic generator (according to the prior art). Most photovoltaic generators comprise at least one panel, itself comprising photovoltaic cells connected in series and/or in parallel. A plurality of groups of cells can be connected in series to increase the total voltage of the panel; a plurality of groups of cells can also be connected in parallel to increase the intensity delivered by the system. In the same way, a plurality of panels can be connected in series and/or in parallel to increase the voltage and/or the amperage of the generator according to the application.
FIG. 1 shows a photovoltaic generator comprising two parallel branches, each containing three groups of cells 2. In order to guarantee the electrical safety of the photovoltaic generator, non-return diodes 3 and bypass diodes 4 can be provided. The non-return diodes 3 are connected in series to each parallel branch of the generator in order to avoid the flow in the cells of a negative current arriving from the load or from other branches of the generator. The bypass diodes 4 are connected in anti-parallel to the groups 2 of cells. The bypass diodes 4 enable the separation of a group 2 of cells presenting a deficiency or a shadowing problem and solve the problem of hot spots.
The maximum voltage of the generator corresponds to the sum of the voltages of the cells of which it is comprised, and the maximum current that the generator can deliver corresponds to the sum of the maximum currents of the cells. The maximum voltage VOC of a cell is reached for a cell on no load, i.e. for a zero delivered current (open circuit) and the maximum current ICC of a cell is reached when its terminals are short-circuited, i.e. for a zero voltage on the terminals of the cell. The maximum values VOC and ICC depend on the technology and the material used to implement the photovoltaic cell. The maximum value of the current ICC also depends strongly on the level of insolation of the cell. A photovoltaic cell thus presents a non-linear current/voltage characteristic (IPV, VPV) and a power characteristic with a maximum power point (MPP) which corresponds to optimum voltage values Vopt and optimum current values Iopt. FIG. 2 shows the current/voltage (IPV, VPV) and power/voltage (PPV, VPV) characteristics of a photovoltaic cell with its maximum power point (identified by PPM in the figure). Similarly, a photovoltaic generator will present a non-linear current/voltage characteristic and a power characteristic with a maximum power point. If a part of the cells is shadowed, or if one or more cells of the group is defective, the maximum power point MPP of this group will be displaced.
It is known to optimize the operation of a photovoltaic generator through the use of a command to search for the maximum power, known as a Maximum Power Point Tracker (MPPT). An MPPT command of this type can be associated with one or more static converters (CS) which, according to the applications, can be a direct-current/alternating-current (DC/AC) converter or a direct-current/direct-current (DC/DC) converter. FIG. 1 thus shows a DC/AC static converter 8 connected to the output of the generator and collecting the electrical energy produced by all of the cells of the generator to deliver it to a load. According to the requirements of the load, the converter can be made to increase or reduce the output voltage and/or to invert the output voltage. FIG. 1 thus shows an MPPT command 6 associated with the converter 8.
The MPPT command 6 is designed to control the converter(s) 8 in order to obtain an input voltage which corresponds to the optimum voltage value Vopt of the photovoltaic generator (PVG), corresponding to the maximum point of the power characteristic. The maximum power point depends on a plurality of parameters that are variable through time, notably the insolation present, the temperature or the ageing of the cells or the number of cells in a functional state.
In this way, the output of the photovoltaic generator is not too adversely affected by the malfunction or shadowing of certain cells. The electrical output of the generator depends directly on the state of each photovoltaic cell.
The power delivered by the photovoltaic generator will vary as a function of the insolation. Notably, not one but two or three or even more converters can be used as a function of the power. The method consists in adapting the number of (cell or phase) converters as a function of the changes in the power produced by the PVG. In fact, the use of a single converter is not necessarily advantageous in order to manage substantial power variations, the conversion output being adversely affected. The output of a power converter constituted on the basis of a single phase (or of a single converter) reduces when the PV power supply is maximum, whereas the structure including three converters has a tendency to maintain a virtually constant output regardless of the delivered PV power. This will result in a greater transfer of energy to the battery.
FIG. 3 shows an arrangement of this type, including at the output of the PV cells three CS (which, in this case, are BOOST converters). These converters are actuated as a function of the generator power in relation to the peak power of the device (Ppeak). In a known manner, when the power delivered by the PVG is less than or equal to one third of the Ppeak, one CS is used; when the power delivered by the PVG is between ⅓ and ⅔ of the Ppeak, 2 CS are used, and when the power delivered is greater than ⅔ of Ppeak, 3 CS are used.
In the event of meteorological changes, the number of converters involved will therefore change, since the power generated by the PVG will vary. These changes may be numerous in the course of one day and over the service life of the PVG. Numerous changes impose stresses on the components, notably those of the converters, which causes the devices to age.
There is therefore a need to reduce the ageing of the components of the PVG.